It is often desired to reclaim vulcanized rubber. The vulcanized rubber is generally in the form of a manufactured article such as a pneumatic tire, industrial conveyor or power transmission belt, hose and the like. Scrap pneumatic tires are especially large source of such vulcanized rubber.
The vulcanized rubber is conventionally broken down and reclaimed by various processes, or combination of processes, which include physical breakdown, grinding, chemical breakdown and devulcanization and cryogenic grinding. If the vulcanized rubber contains wire or textile fiber reinforcement then it is generally removed by various processes which might include a magnetic separation, air aspiration and/or air floatation step.
In this description, the terms "reclaim" and "reclaimed rubber" are used somewhat interchangeably and relate to devulcanized rubber which is more completely hereinafter described.
It is important to appreciate that devulcanized reclaim or reclaimed rubber relates to rubber which has been vulcanized followed by being substantially devulcanized.
The resultant reclaim rubber is a polymeric material which has somewhat the appearance of unvulcanized rubber but has important differences and properties therefrom. First, it is a polymer which is, in essence, a partially vulcanized rubber composed of a mixture of polymer units of various and numerous constructions different from either unvulcanized or vulcanized rubber.
Secondly, the reclaim rubber, unlike conventional unvulcanized rubber, is also a complex mixture of largely unknown polymer(s), of compounding ingredients, possibly bits of textile fiber, and the like.
It has been observed that, after adding sulfur and accelerator to reclaim rubber, followed by its revulcanization, the resulting physical properties, such as tensile and elongation, are usually lower than the corresponding properties of the original vulcanized rubber from which it was derived. It has also sometimes been observed that exposed edges of bales or slabs of reclaim rubber have tended to curl up, apparently a result of oxidation degradation which was probably due to a deficiency of antidegradants which would normally have been adequately present in unvulcanized, compounded rubber.
It is further important to appreciate that the constitution of reclaim rubber composition itself is also dependent on both the reclaim process used and the actual source or composition of vulcanized rubber from which the reclaim is derived.
In manufacturing processes, reclaim rubber, usually along with the addition of other compounding ingredients, is conventionally mixed with unvulcanized rubber to form a compounded rubber mixture which, in turn, is shaped and cured to form a manufactured article.
Reclaim rubber, prior to further compounding, has conventionally been produced in the form of a slab or bale which is prevented from sticking to adjacent or stacked bales by dusting the slabs or bales with clay, soapstone or other anti-block agents. This has sometimes created an undesirable dust contamination in the work area. A usually more effective dust-free method of protecting reclaim rubber has been to package the reclaim in a manner similarly used for synthetic or natural unvulcanized rubber, namely, to wrap each bale in a protective film such as polyethylene film. Polyethylene packaging films, however, generally do not disperse easily in a rubber compound during mixing except somewhat at elevated temperature.
Unvulcanized rubber is typically compounded in at least one and usually a series of mixing stages.
A mixing stage relates to when a rubber compound is in a mixing machine and for duration of the time it is in the machine. There may be several mixing steps within the stage in which rubber or compounding ingredients are stepwise added and mixed.
For example, unvulcanized rubber can be compounded by mixing rubber and compounding ingredients in 1 to 5, or more, sequential mixing stages in which at least a portion of the mixed materials of a preceeding stage are mixed with additional materials in a succeeding mixing stage. More than one mixer or mixing machine may be used. As previously pointed out, the mixing may be done in stages where more than one mixing step is conducted in a stage. When two or more mixing stages are used, the temperature of the mixture in the first stage is generally substantially higher than the temperature of the mixture in the final or last stage of mixing where the curative is added.
For example, conventionally unvulcanized synthetic and/or natural rubber is at least added in the first and optionally, a portion thereof also added in a later stage; compounding ingredients for said unvulcanized rubber can be added in any or all of the stages, as may be desired, whereas a curing agent(s), other than accelerators, is added only in the last stage.
Thus, one stage of mixing, which may in reality be a series of steps, might be conducted all in one mixer. In this case, the curative would conventionally be added in the last step.
Reclaim rubber, although usually not considered in the same sense of a conventional compounding ingredient such as carbon black, curatives, processing aids, antidegradients, etc., is added in the mixture in the same manner as such conventional compounding ingredients.
It is important to appreciate that the reference to staged mixing of the unvulcanized rubber relates to a series of mixes. When mixing a batch in a mixer, it is generally desired that the volume of the mixer is practically full of the respective mixture itself. Therefore, in a staged mixing method, for example, unvulcanized rubber may be mixed in a first mixer (or stage) and the resultant mixture removed from the mixer itself. Then, only a portion of the mixed rubber might be added to a second mixer to which is then added additional compounding ingredients with the total mixture essentially or practically filling the mixer itself. The unused portion of the mixed rubber from the first stage can be saved for further use in another mixer.
Upon removal of the resultant mixture from the second mixer, only a portion of the second mixture might be added to a third mixer in which a similar mixing process is carried out. Such staged mixing is then continued through a sufficient number of stages, or mixers until all of the desired ingredients have been added and mixed to form the final compounded rubber.
An important difficulty experienced in such sequential mixing relates to the relatively lower temperatures required in the last or final mixing stages, in which the compounded rubber contains its curing agent(s), to prevent premature vulcanization.
Therefore, polyethylene film packaged reclaim rubber is typically introduced in an earlier mixing stage such as first or second stage, where the temperature is higher so that the film will more adequately disperse. This can be a particular disadvantage and require more mixing power for the overall mixing process because of the greater rubber volume or mass involved in the sequential stages, as compared to a reduced power requirement if the reclaim could be added in a later or last mixing stage. Such mixing power utilization is therefore of a relatively low mixing and energy efficiency.
Recognized alternatives are (1) to remove the polyethylene film from the reclaim bales or slabs and add the unwrapped reclaim directly to the last mixing stage or (2) to add dusted or soapstoned reclaim bales or slabs to the last mixing stage with an attendant potential dust contamination problem in the work area.
Therefore, it is desired to provide reclaim rubber as a package which can be useful as an ingredient for compounding unvulcanized rubber and to provide the resultant compounded rubber.
Such a package would desirably provide a more efficient use of mixing time and mixing volume by enabling the reclaim rubber to be added in a later mixing stage.